Fluorine resins such as polytetrafluoroethylene (hereinafter abbreviated as PTFE) are plastics having superior anti-sticking, heat resistant, chemical resistant, and electrical insulating properties and, making use of these advantages, articles shaped from metals having PTFE coatings are widely used in home kitchenware and in industrial parts such as fixing rollers of copying machines.
However, fluorine resins typified by PTFE and tetrafluoroethylene/hexafluoropropylene copolymer have the disadvantage that their adhesion to metals is very poor, and special methods are required to achieve bonding between fluorine resins and metals. The methods conventionally employed in bonding fluorine resins to metal substrates are roughly classified into two types, i.e., chemical and mechanical methods, in accordance with the mechanism of bonding. A typical example of the former method is primer coating and the latter method is typified by etching.
In the primer coating process, the surface of a metal of interest is roughened by a suitable method such as sand blasting and is then coated with a primer (i.e., adhesive layer), which is in turn coated with a fluorine resin layer. In other words, the metal is bonded to the resin with the aid of an intervening primer.
In the etching process, the surface of a metal substrate, say aluminum, is etched either electrochemically or chemically to produce fine asperity on the surface, which then is coated with a fluorine resin. In the etching method, aluminum is bonded to the resin by an anchor effect provided by the etching.
Of these two methods, the etching process is superior because it achieves stronger adhesion between metals and fluorine resins, and if fluorine resins are coated onto flat metal sheets, they can be formed into a desired shape.
However, the conventional methods of bonding fluorine resins to aluminum have an economic disadvantage in that in addition to the steps of coating and sintering the resin, an additional step (i.e., the step of applying a primer coating in the primer coating process, and the step of roughening the surface of aluminum in the etching process) are required. Another problem with the conventional bonding techniques is that it is very difficult to provide equipment adapted to the bonding of a fluorine resin onto a thin aluminum sheet in view of the requirement of roughening its surface by etching, sand blasting, or other appropriate method.